Horizontal electrophoresis has been an enduringly popular choice for separation of nucleic acids and proteins and offers several advantages over vertical electrophoresis. Generally, horizontal gels used as separation media are easier to cast than vertical gels. Further, thicker gels can be provided in a horizontal format than in a vertical system, and a lower concentration of separation medium, such as agarose, can be used in horizontal gels than in vertical gels allowing for better separation of high molecular weight molecules. Also, in a horizontal format, sample wells may be provided within the interior of the gel as well as at the top edge, increasing the number of samples that can be loaded per gel.
The typical system for casting horizontal gels utilizes a gel-casting tray with a flat bottom surface, two opposing parallel sides and two open ends that must be blocked at set points to retain the separation medium until the medium solidifies, resulting in a gel of limited dimensions. The open ends are usually sealed with an adhesive tape or sealed by the placement of casting gates in pre-cut grooves in the sidewalls and bottom surface of the tray. Whatever method is used, the gel is formed only in specific, predetermined lengths.
Several horizontal gel systems, in particular, catalog numbers HG-12, HLB-12, HG-20 and MG-10, available through Tyler Research Corporation, Alberta, Canada, utilize an adjustable gate-placement design. The design allows a casting gate to be placed into one of three placement grooves at pre-set lengths. Although the design increases the flexibility of a system in that it provides for some choice of length to be made, it is still limited to pre-determined lengths. Thus, there is a need for a horizontal gel-casting device that permits the selection of length from along a continuum of lengths.
A further disadvantage of systems with pre-determined gate settings is that each setting requires a groove cut in the sidewalls and the otherwise smooth bottom surface of the casting tray for gate placement. Because the grooves are machined for a tight fit with the casting gate, additional grooves add to the cost of manufacturing. Moreover, when a relatively longer gel is poured, the grooves cut in the bottom surface of the casting tray cause variation in gel thickness leading to possible distortion of the separation pattern in the gel.
Another limitation associated with using trays with only preset gel dimensions is that the preparation of a gradient gel, in which the concentration of agarose or other ingredients is varied from one end of the gel to the other, is not possible using existing casting tray systems with placement of casting gates only at the ends of the trays, and is not practical using systems with only a few pre-determined gate settings along the length of the tray. Gradient gels are advantageous when separating specimens containing macromolecules over a wide range of sizes. There is a need for a horizontal gel casting system that permits incremental increases, as desired, in the concentration of the separation medium along the length of the gel.